In 3GPP LTE networks, a base station (i.e. evolved NodeB, eNB) of a Radio Access Network (RAN) transmits data and signaling between a core network (CN) and User Equipment (UEs) located within the base station's coverage area.
In addition to normal base stations, relay base stations (relay nodes) were introduced in Rel-10 of the 3GPP standards documentation to provide coverage extension within the cell(s) operated by base stations, as a tool to improve, for example, the coverage of high data rates for user equipment, temporary network deployment, cell edge throughput and/or to provide coverage in new cell areas. Relaying is realised by having a relay node wirelessly connected to a donor base station (DeNB). In addition to serving its own ‘donor’ cell, the DeNB serves the RN (and hence any user equipment connected to this relay node), via a modified version of the Evolved Universal Terrestrial Radio Access (E-UTRA) radio interface. The modified interface is referred to as the ‘RN-Un’ interface. The 3GPP standards documentation defines in Section 4.7 of TS 36.300 v11.3.0 (the content of which is herein incorporated by reference) the architecture of RNs and the way in which they establish connections with their donor base station. Mobile RNs (MRNs) are also included in Rel-11 as a study item and the deployment use case is limited to high speed trains where the relay node in mounted on and moves with the train.
Each RN is provided with many aspects of a base station's functionality and is therefore capable of acting as a base station serving user equipment in its own ‘relay’ cell via a wireless interface referred to as the ‘RN-Uu’ interface. From the perspective of the user equipment in the relay cell, therefore, the RN appears to be a conventional LTE base station. Typically an RN will be serving multiple UEs so the aggregated data for all these UEs must pass over the ‘RN-Un’ interface. In addition to the base station functionality, however, the RN also supports a subset of the UE functionality including, for example, many aspects of the physical layer, layer-2, radio resource control (RRC), and non-access stratum (NAS) functionality, to allow it to connect wirelessly to the DeNB.
The DeNB is capable of handling communication ‘directly’ to and from user equipment camped in its own cell via a conventional ‘Uu’ interface between the DeNB and the user equipment. The DeNB is also capable of handling communication ‘indirectly’ with user equipment camped in the relay cell, via the ‘RN-Un’ interface, the RN, and the ‘RN-Uu’ interface.
As mobile telephones (or other user equipments) move around in the area covered by the communication system, they are handed over from one cell (i.e. a serving cell operated by either a base station or a relay node) to another suitable cell, depending on signal conditions and other requirements, such as a quality of service requested by the particular mobile telephone, the type of service used, overall system load, and the like. A trigger for handing over a mobile telephone to a new cell may be based on signal measurements performed by the particular mobile telephone with respect to the current and/or the neighbour base station cell(s). The measurements might comprise measuring the strength of cell reference signals (CRS) or channel state reference signals (CSI-RS) broadcast by the neighbouring cells. Depending on the technology used, the mobile telephone might also determine signal conditions in any given cell by measuring either one of a Reference Signal Receive Power (RSRP), a Reference Signal Receive Quality (RSRQ), Received Signal Strength Indicator (RSSI) and a Received Signal Code Power (RSCP) of that cell—depending on the access technology used in that cell.
When the signal measurements indicate more favourable signal conditions in a different cell than the current serving cell, the mobile telephone informs the serving base station about the favourable signal conditions, based on which indication the base station may initiate handover procedures to this new cell. Therefore, handover decision and target cell selection are made by the serving base station by taking into account the indication received from the mobile telephone. The type of triggers and the related measurements are detailed in section 5.5.4 of the 3GPP TS 36.331 v11.1.0 standard, the content of which is herein incorporated by reference. In particular, the above standard defines measurement report triggering related to eight different event types (Events A1 to A6, B1, and B2) that the base station may configure for user equipment within its cell(s). In summary, each event relates to a scenario in which measured signal conditions meet a predetermined criteria. When a particular event occurs (i.e. measured signal conditions meet the associated predetermined criteria) transmission of an event report describing the event type and associated parameters is triggered. For example, an event may occur if measured signal conditions in the mobile telephone's serving cell (and optionally also modified by a pre-defined offset value) become worse (or become better in a neighbouring cell) than a pre-defined threshold. 3GPP has not introduced any events related specifically to relay nodes because, from the network point of view, relay nodes are treated as base stations (and indeed relay node cells appear to be base station cells for the mobile telephones).
Further details of the overall mobility sequence are described in section 10.1.2 of the 3GPP TS 36.300 standard, which describes the configuration of measurements by the base station and the subsequent triggering of handover.
Recently, 3GPP introduced the possibility of direct, device-to-device (D2D) communications between mobile telephones that are in each other's proximity. In case of D2D communications, user data is exchanged between the two (or more) mobile telephones without routing it via the radio access network and the core network, whilst maintaining a control link between each involved mobile telephone and their respective base stations. In LTE networks, D2D communications are thus carried out under continuous network control and only whilst the involved mobile telephones are operating within the network's coverage. The D2D approach results in a more efficient usage of the valuable radio resources available to the base station(s). Example D2D communications have been presented in 3GPP document no. S1-113344 titled “Feasibility Study for Proximity Services”.
Direct communication channels between mobile telephones may also be beneficially used to implement a UE-based relaying function when one mobile telephone relays data and signalling for another mobile telephone (i.e. to/from a serving base station). The relaying mobile telephone is called UE Relay (UE-R) throughout this document. Such a UE-based relaying function might beneficially further improve the cell coverage of a serving base station and/or load balancing of the LTE network.